New directors are required to complete the following forms:
Form AP01 - appointment (Companies House)
Form SA01 - registration for Self Assessment and getting a tax return.
An agent will require agent authority, this is via online services or Download a form.
Useful guides on this topic:
Do I need to file a tax return?
Do I need to file a Self Assessment tax return? What if I am a director? Can HMRC make directors file tax returns? When should I file a Self Assessment return?